Plants really do make methane
29 November 2007
Chinese chemists have confirmed the contentious finding that plants can emit methane, a potent greenhouse gas.
Zhi-Ping Wang of the Chinese Academy of Science, Beijing, and colleagues studied the methane emissions of 44 species of plants from the temperate grasslands of Inner Mongolia. While none of the 35 herbaceous species tested seemed to produce methane, seven out of nine shrub species did emit the gas.1
Frank Keppler, from the Max Planck Institute f or Nuclear Physics, Heidelberg, Germany, shocked the scientific world with his January 2006 Nature2 publication claiming that the world's plants, previously seen as a greenhouse gas sink thanks to their CO2 uptake, actually emit millions of tonnes of methane. As a greenhouse gas, methane is 20 times more potent than CO2.
However, while Keppler's studies of individual leaves suggested that plants produced up to 30 per cent of the world's methane, subsequent work by Tom Dueck of Wageningen University, the Netherlands, found that some of the same plants used in the original experiments - basil, wheat and maize - emitted virtually no methane at all.
Dueck grew his plants in a 13C-enriched environment, to test whether they subsequently produced 13CH4, which would be clearly distinguishable from the background 12CH4 in the air. Dueck's whole plants produced negligible methane, and he suggested Keppler's study hadn't ruled out the possibility that residual atmospheric methane was slowly leaking out of intercellular spaces in the leaves.
However, Dueck only tested herbaceous plants - which, as Zhi-Ping's latest results suggest, do not emit methane.
'To grow woody shrubs in a 13C-enriched environment would have taken too long and been too costly,' Dueck explained to Chemistry World. 'Whereas herbaceous plants can be grown and tested in a few weeks.'
'Zhi-Ping's study is a nice piece of work - it looks at the problem using a different approach, classifying a large number of species into different plant types,' he added.
Zhi-Ping also found that several plants continued to emit the gas in methane-enriched atmospheres. 'Testing methane emissions in atmospheres with different background levels of the gas support their results,' said Dueck. However, the mechanism that the plants use to produce methane remains to be explained.
Zhi-Ping concludes that, as woody shrubs are rare in Inner Mongolia, the overall methane emissions will be negligible in that region - but that plant methane production could play a larger role in upland wooded areas. 'As Zhi-Ping has measured it, the methane plants produce wouldn't be a large figure, globally speaking,' Dueck pointed out. 'But it's certainly worth further investigation by atmospheric modellers.'
James Mitchell Crow
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1. Z-P Wang et al., Environ. Sci. Technol., 2007 (DOI: 10.1021/es071224L)
2. F Keppler et al, Nature, 2005, 439, 187 (DOI:10.1038/nature04420)
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